DESCRIPTION (From applicant's proposal): Sounds from the ear-known as otoacoustic emissions (OAEs)-are now widely used as clinical diagnostic and screening tools. Nevertheless, relatively little is known about the physical mechanisms that generate and shape them. Thus, although otoacoustic emissions offer a promising noninvasive window on the biophysics of cochlear function, the clinical use of OAEs is currently limited to a largely qualitative assay of hearing threshold. Realizing the considerable potential of OAEs as nonivasive probes of cochlear state requires understanding the role of cochlear nonlinearity, amplification, and middle-ear transmission in shaping the ear-canal spectrum of these sounds. This project focuses on specific issues that probe the mechanisms underlying the generation and propagation of otoacoustic emissions. By combining ear-canal measurements of OAEs in cat with simultaneous measurements of the responses of auditory-nerve fibers and middle-ear transmission, the project explores a number of issues central to any quantitative interpretation of OAE measurements. The measurements will be used to develop and test a comprehensive model of cochlear emissions that will allow ear-canal measurements to be interpreted directly in terms of cochlear mechanics. The model will be combined with measurements of distortion-product emissions to estimate the spatial dependence and gain of the "cochlear amplifier" and other important characteristics of cochlear physiology. The project lays the necessary groundwork both for improving the clinical utility of current OAE measurements and for developing new, more functionally interpretable measures of cochlear state.